Rates of prediabetes and diabetes are increasing in the pediatric population, due to the epidemic of childhood obesity. Epidemiologic studies have linked prediabetes in childhood with a higher risk of type 2 diabetes in adulthood. Because children with prediabetes represent a high-risk group of individuals who may benefit from early identification and intervention, there is increasing interest in the use of biomarkers previously validated in adults for pediatric populations. The proposed study will evaluate the longitudinal test performance of an array of conventional biomarkers of glycemia, including Hemoglobin A1c (HbA1c), and novel metabolomic biomarkers for identifying progression of glucose tolerance (normal to prediabetes or prediabetes to diabetes) in an overweight and obese pediatric cohort. In addition, a feeding study will also be conducted to examine variability in metabolomic biomarkers associated with abnormal glucose tolerance in response to macronutrient diet composition. The specific aims of this study are the following: Specific Aim #1: Assess whether conventional biomarkers of glycemia longitudinally predict progression of glucose tolerance (normal to prediabetes or prediabetes to diabetes) in a cohort of overweight/obese children. Specific Aim #2a: Identify novel metabolomic measures that longitudinally predict progression of glucose tolerance (normal to prediabetes or prediabetes to diabetes) and insulin resistance in a cohort of overweight/obese children. Specific Aim #2b: Evaluate the variability in metabolomic biomarkers associated with abnormal glucose tolerance in response to high carbohydrate and high polyunsaturated fat diets in an overweight/obese adolescent population. Application of the science of metabolomics to human disease risk is a major focus of the NIH's Roadmap for Medical Research. Although an increasing number of metabolomic studies have been conducted in adults, few studies have been conducted in children. This will be one of the first longitudinal studies to evaluate the predictive capacity of a comprehensive set of conventional and novel biomarkers for identifying adolescents with insulin sensitivity and progression of glucose tolerance status. The proposed research project represents a unique multidisciplinary collaboration of investigators with expertise in pediatric obesity and diabetes, diabetes epidemiology, metabolomics, human nutrition, and biostatistics, and will provide important information relevant for future pediatric screening policy, insights into the pathogenesis of prediabetes/type 2 diabetes during adolescence, and information on specific dietary interventions and their impact on the metabolomic 'at risk' profile.